Battery impedance measurements can be a useful indicator of a number of battery characteristics. Accurate battery impedance measurements are thus important in many applications.
One methodology for measuring the impedance of a battery involves pulsing the battery with a current of known magnitude. The pulse can be either injected into or drawn from the battery. The voltage of the battery is measured during pulsing and also before or after pulsing. The battery impedance can then be determined by dividing the difference between the battery voltage measurements by the magnitude of the current pulse. However, for this battery impedance measurement methodology to be accurate, any other current being drawn from or applied to the battery must remain constant. Also, no part of the current pulse can be diverted to a load connected to the battery.
To insure the accuracy of a battery impedance test, the battery can be removed from its load (the circuit drawing energy from the battery). However, in many applications, it is impractical to disconnect the battery from its load to perform a battery impedance test. For example, with portable computing devices such as palmtop computers, the battery is constantly in use providing power to memory or clock circuits. The battery cannot be removed without disrupting these activities and losing any data stored in the memory circuits, as well as halting execution of any program currently in use.
Another possibility is to connect an isolating circuit or element between the battery and load. However, known isolating circuits have been found to be impractical for this purpose. In portable computing device applications, for example, it is necessary to maintain a low impedance path between the battery and the load in order to maximize the useful life of the battery and to provide reliable start-up characteristics. Conventional isolating devices fail to maintain a low impedance path between the battery and load, thus shortening useful battery life. Also, active switching devices are difficult to control during start-up. Therefore, these devices are unsuitable for battery impedance testing in such applications.
The present invention provides a circuit topology for stabilizing or latching the current provided by a battery to a load at a constant magnitude. In a preferred embodiment, the circuit topology comprises an inductor connected in series between the battery and the load. The inductor's terminals are connected to the inputs of a comparator to sense a difference in voltage between the terminals indicative of a change in the current drawn from the battery by the load (the inductor current). The comparator output is applied to a current driver circuit supplied from a storage capacitor. Responsive to the comparator output, the driver circuit provides an additional current to the load to return the current drawn from the battery to its latched magnitude. Latching of the inductor current can be enabled or disabled by a current switch connected between the storage capacitor and the current driver.
The circuit topology has particular application to measuring battery impedance. When the circuit is enabled, the current provided by the battery to the load is latched at a constant magnitude. Thereafter, a current pulse applied to the battery is prevented from diversion to the load. Thus, the impedance of the battery can be accurately measured while in-circuit and operating. Further, since a small value of inductance is sufficient for operation of the circuit, the series impedance between the battery and the load remains low and start-up characteristics are not affected significantly.
Additional features and advantages of the present invention will be made apparent from the following detailed description of a preferred embodiment, which proceeds with reference to the accompanying drawings.